The present invention relates to storage and retrieval of digital information on optical disk media using multi-level rather than binary data coding.
The technology of digital data storage using optical memory devices has advanced rapidly since its inception. Commercial uses at present range from compact disks (CDs) which provide remarkably high-quality audio reproduction to computer memories which provide extremely large yet compact data storage capacity. As applied to the requirements of computer mass storage, optical disks have been categorized according to the ease with which digital data can be written onto them. Optical read-only memories (OROMs) such as compact disk ROMs (CD-ROMs) have data written onto them before they leave the factory; write-once read-many (WORM) disks allow the user to write data onto them once and read that data indefinitely; erasable optical disks permit the user to write and read data with the same flexibility as magnetic storage media such as floppy and hard disks.
One of the reasons that optical storage media have increased in popularity is the very large capacity they provide in a small volume. For example, a single 12-cm diameter CD-ROM stores about 550 megabytes of data while 36-cm diameter media may soon store 10 gigabytes on a single disk side. Even with such large capacity, some applications still strain the capability of optical media; it has been anticipated that 400 megabytes will be necessary to store the data from only a single day's operation of a modern space problem. Besides coping with these massive-capacity applications, optical disks which can store data at a higher density can be smaller, lighter and cheaper than other memory devices.
Most optical disk systems employ apparatus such as that shown in FIG. 1. The optical disk 1 which generally comprises a transparent substrate 3 and a thin recording film 5 deposited on the substrate is often illuminated from below by light from a laser. There is sometimes a reflective layer (not shown) of a metal such as aluminum deposited on the recording film 5. The linearly polarized light from the laser is collimated and shaped by beam-forming optics 12 and directed to the disk by a beam-splitter 14. The beam-splitter 14 is of the polarizing type and may be arranged as shown in the figure to reflect light polarized in the direction of the laser. A quarter-wave plate 16 converts the linearly polarized light into circularly polarized light which is focused by an objective lens 18 to a very small spot on the disk 1. The objective lens is axially and transversally movable to maintain proper focusing and positioning of the light on the disk. Because the size of the focused spot is of the order of one micrometer in diameter, each disk can store a very large quantity of information.
Some of the light focused on the disk is reflected. This light is collected by the objective lens 18 which directs it back through the quarter-wave plate 16. Since the light reflected is circularly polarized, the plate 16 converts that light to the linear polarization which is perpendicular to that emitted by the laser 10. Light of that polarization is transmitted by the beam-splitter 14 and thus the laser 10 is isolated from the reflected light. The light reflected from the disk 1 which is transmitted by the beam-splitter 14 is finally focused by another lens 20 divided by another beam-splitter 22, and detected by two sets of detectors 24 and 26. The first set of detectors 24 is used to detect the information stored on the disk and to derive signals for correcting the axial position of the objective lens 18. The second set of detectors 26 is used to derive signals for correcting the transverse position of the objective lens.
It should be understood that the apparatus shown in FIG. 1 is only illustrative of a conventional optical disk memory system. The necessary data and position signals can be derived from only a single set of detectors, rendering the beam-splitter 22 unnecessary. Also, the polarizing beam-splitter 14 may be arranged to transmit the light emitted from the laser 10 and reflect the light reflected by the disk 1.
Information is usually stored on the disk in the form of pits in the recording film 5. As illustrated in FIG. 2, the pits 7 can consist of holes, which are arranged in tracks 9, in the recording layer 5. The holes are usually formed by an intense laser beam focused onto the layer which is formed of a material which absorbs the laser light and ablates or melts as a result of heating caused by the energy absorbed. The intensity of the light reflected from the disk is modulated by the presence or absence of the pits. The layer 5 may be composed of tellurium alloys, bubble-forming materials, multilayer optical cavities, colloids, microtextured absorbers or organic dyes. The pits are written by a relatively high power laser beam, for example 10-30 mW, while they are read by a low power beam, typically 0.5 mW. Both reading and writing can be performed by a single semiconductor laser, such as those of gallium arsenide, emitting at wavelengths in the near-infrared spectrum between about 0.7 micrometers and 1.6 micrometers.
The intensity of the light reflected from the disk is modulated by the pits in at least two ways. As described in U.S. Pat. Nos. 4,161,752 and 4,475,183, light reflected from the pits 7 interferes with light reflected from the disk surface adjacent to the pits. The disk surface adjacent the pits is either another data track having a different depth of pit read by a second laser of appropriate wavelength or an unmodified region of the disk. Since the phase of the light reflected from the bottom of the pit is different from the phase by the light reflected from the adjacent surface because the pit-reflected light travels farther before being reflected, interference occurs. The phase difference between the reflected light bemas is adjusted to cause destructive interference, resulting in the presence of a pit being detected as a reduction in reflected intensity.
Another way that the reflected intensity can be modulated by the pits arises from the scattering of light caused by the pits. Light is reflected from the unmodified flat disk surface in a substantially constant direction toward the objective lens 18. On the other hand, light is reflected from the pits in a multiplicity of directions, so the amount of light collected by the objective lens is always less when a pit is illuminated. Again, the presence of a pit is detected as a reduction in reflected intensity.
According to these methods, the intensity of reflected light takes on either one of two main values which are arbitrarily assigned to represent the ONE and ZERO of a binary digital encoding system. Since one of the major advantages of optical storage media is their high data storage capacity, using only a binary system limits that capacity. The system disclosed in U.S Pat. No. 4,161,752 is directed to increasing the disk capacity by placing the data tracks closer together However, to avoid cross-talk between the adjacent tracks, the system disclosed includes two read lasers of differing wavelength. This increase in complexity and cost could be avoided by an optical disk in which data is encoded by a multi-level digital system.